This invention relates to a medium for electrographic printing which yields quality images and which comprises a paper substrate having sufficient wet burst strength such that the electrographic medium is suitable for outdoor use. In still another aspect, the present invention relates to pasted displays, and in particular outdoor signs, made with the electrographic medium of the present invention.
The use of electrographic processes is well known in the art. In these processes, the electrostatic latent image is produced directly by "spraying" a charge onto an accepting dielectric surface in an imagewise manner. Styli are often used to create these charge patterns and are arranged in linear arrays across the width of the moving dielectric surface. These processes require an apparatus as described, for example, in U.S. Pat. Nos. 4,007,489; 4,569,584; 4,731,542; 4,808,832; and European Patent Publications 444,870 and 437,073, the subject matter of which is hereby incorporated by reference.
The use of electrographics to generate images suitable for use on billboards and other outside exhibitions is also known to the art. For example, European Patent Publications 444,870 and 437,073 disclose processes for making large size, full color images by electrographic means. The processes described in the foregoing European patent publications involves transferring an image from an imaging sheet to a separate receptor sheet. The receptor sheet is selectively chosen to have the required properties for a final print useful for outdoor signing. The transfer to a final imaging sheet was found necessary in order to overcome the problems that typical paper substrates have in the electrographic art, i.e., a lack of water and UV resistance necessary for outdoor signing.
Indeed, electrographic imaging has not been a viable imaging technique for the outdoor market because the papers available cannot withstand either the pasting techniques currently used or outdoor weather conditions for extended periods of time. Insufficient wet strength of the current electrographic papers causes the paper to tear during pasting. Furthermore, separation of the dielectric layer from the conductive layer often occurs upon application of pasting solutions.
As a result, the current outdoor market is presently being served predominantly by silk screen, lithography, and manual or mechanical painting techniques. Such methods are slow and very uneconomical, particularly for short run (1 to 10 runs) printing. Direct imaging on water insensitive electrographic film has been found feasible, but such material is unsuited for mounting by the typical methods employed in the outdoor market industry. Imaging directly onto fabrics for such applications has been proposed, for example, see Japanese Patent Application 21-18665, but the materials used have insufficient opacity.
Accordingly, there is a real need in outdoor signing and other posting applications, for a weatherable, pastable electrographic medium which provides high image quality if electrographic imaging is to be useful. It would be advantageous to develop such a pastable paper substrate since electrographic imaging can quickly and economically print individual billboards provided the paper substrate does indeed meet the wet strength (and wet expansion) requirements necessary.
It is therefore an object of the present invention to provide a novel electrographic medium which is pastable and which provides high quality imaging.
Yet another object of the present invention is to provide one with an electrographic medium having sufficient coating adhesion of the dielectric layer, as well as wet expansion and wet strength properties, to withstand aqueous pastes and outdoor conditions and thereby provide an excellent medium for outdoor signing.
It is still another object of the present invention to provide one with a process for making a posterboard useful as an outdoor sign by electrographic imaging, which posterboard overcomes the problems of the prior art.
Yet another object of the present invention is to provide one with a novel pastable display medium having high image quality, which image has been printed by electrographic means.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.